Angle resolvers for comparing a controlled angle to a reference angle in order to give an indication to error or to provide an error signal for use in a feedback control system have existed for a considerable time.
Such systems have used a variety of means of sensing angles and comparing them, such as the crossed coils of the goniometer and various systems employing selsyns to both indicate angles and angular deviations from a reference angle. Also in this category of angle measurement devices might be mentioned the movable scales provided on some compasses to permit direct reading of angle of deviation from a desired heading.
More recently, of course, the development of compact, lightweight, and relatively inexpensive electronics in more-or-less integrated form has made possible the rapid and simple measurement and calculation of the difference between a pair of angles which are presented to the electronic angle resolver in the form of electrical analog signals representing the magnitude and direction of the angles. Such entirely electronic angle resolvers have in particular been proposed for use in the construction of automatic pilots which can steer a steady course in a ship or boat, for example, once given the desired heading.
Such electronic angle resolvers may operate in conjunction with an electronic compass which provides electrical analog signals representative of the sine and cosine of the angle of heading of a boat. Means may also be provided for setting a desired heading, which heading becomes an additional electrical analog signal entered into the electronic angle resolver.
An electronic angle resolver may employ anyone of several approaches available for resolving the angle of heading or its deviation from a desired heading. A resolver may be constructed to make use of any appropriate one of the trigonometric identity formulas depending upon the trigonometric functions of the angles readily available or derivable. The choice among the various techniques depends at least in part on cost, complexity, reliability and angular resolution needed.
If the angular resolver is used as part of a feedback control system or autopilot, all that is needed is the angular deviation (sign and magnitude) of the actual heading as measured from the desired heading. Similarly, many other angle deviation problems in navigation, fire control of weapons, remote positioning of devices for automated production, etc. can be solved by the use of circuitry which determines the deviation of one angle from another.
One attractive approach to the solution of such problems centers around the trigonometric identity sin (A-B)=sin A cos B-cos A sin B. For example, if A represents an unknown angle to be determined by comparison with a reference angle B, the compass used to indicate A may be easily so arranged as to generate electrical analog signals which are represented of the sine and cosine of A. The apparatus may then be adapted to accept through a manual keyboard or otherwise, a reference angle B against which angle A is to be measured, with the deviation therebetween represented as an electrical analog output signal. Such an angle resolver would be especially useful in an autopilot, for example. In fact, electronic angle resolvers for determining angular deviations are in existence. Unfortunately, their cost and complexity have limited their acceptance. The present application discloses new and useful improvements which significantly lower their cost and improve their reliability without impairing accuracy.
In particular, although it is relatively simple to construct an electronic compass which provides analog voltages which accurately represent the sine and cosine of the angle being measured by the compass, somewhat more complexity is encountered when it is necessary to provide an output voltage which is an accurate analog representation of sin (A-B)=sin A cos B-cos A sin B, for the derivation of values from such an identity relationship requires the cosine and sine of B together with means to form the two products and to derive the algebraic sum thereof. Although the problem is easily within the scope of a microcomputer, it is desirable to utilize still simpler components to derive the voltage analog of sine (A-B). In this way, the expense and complexity of an electronic angle resolver can be significantly reduced and reliability improved.